1. Field of the Invention
The present invention relates to a filter operating with acoustical surface waves, a so-called SAW filter, including two acoustical paths each having one unidirectional input transducer, one unidirectional output transducer, and a reflector structure therebetween.
Such SAW filters are known, for instance, from U.S. Pat. No. 5,475,348 to Hode et al and from an article entitled "A New Compact Saw Low Loss Filter for Mobile Radio" in the IEEE 1993 Ultrasonics Symposium, pp. 105-109. The filters involve small filters with low losses, particularly for mobile radio applications. Those filters combine the advantages of low insertion loss, simple tuning, ease of manufacture and small size, which is of great significance especially for mobile telephones.
However, a disadvantage of a known filter of the type described above is that the extinction of the direct transducer to transducer signals is obtained only at the mean frequency of the filter. The result is therefore not only a transmission curve centered at the mean frequency of the filter but also besides that transmission curve still other transmission ranges, which are centered around frequencies differing from the mean frequency of the filter.
2. Summary of the Invention
It is accordingly an object of the invention to provide a filter operating with acoustical surface waves, which overcomes the hereinafore-mentioned disadvantages of the hereto-fore-known devices of this general type and which improves such a filter in such a way that an extinction of direct transducer to transducer signals is present not only at the mean frequency.
With the foregoing and other objects in view there is provided, in accordance with the invention, a filter operating with acoustical surface waves, comprising two acoustical paths each having one unidirectional input transducer, one unidirectional output transducer, and a reflector structure disposed between the input and output transducers; the input transducers in the acoustical paths being structurally identical, the output transducers in the acoustical paths being structurally identical, and the input and output transducers in each of the acoustical paths being spaced apart by the same distance; one of the input transducers in the two acoustical paths and the output transducers in the two acoustical paths being electrically connected in phase opposition, and the other of the input transducers in the two acoustical paths and the output transducers in the two acoustical paths being electrically connected in phase; and the reflector structures in the two acoustical paths having substantially the same reflection performance, the reflector structure in one of the acoustical paths being shorter than the reflector structure in the other of the acoustical paths by .lambda./2, and the reflector structures in the two acoustical paths being disposed centrally between the input and output transducers, wherein .lambda. is a wavelength at a mean frequency of the filter.
In accordance with another feature of the invention, the transducers are SPUDTs.
In accordance with a further feature of the invention, the reflector structures have electrode strips with widths, and the reflector structures are weighted by varying the widths of the electrode strips.
In accordance with an added feature of the invention, the reflector structures have electrode strips, and the reflector structures are weighted by shifting positions of the electrode strips.
In accordance with an additional feature of the invention, the reflector structures have electrode strips with widths, and the reflector structures are weighted by a combination of varying the widths and shifting positions of the electrode strips.
In accordance with a concomitant feature of the invention, the reflector structures have electrode strips, and the reflector structures are weighted by omissions of the electrode strips.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a filter operating with acoustical surface waves, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.